Proppant having a polyamide imide coating

ABSTRACT

A proppant comprises a particle and a polyamide imide coating disposed on the particle. A method of forming the proppant comprises the steps of providing the particle, providing the polyamide imide coating, and coating the particle with the polyamide imide coating.

RELATED APPLICATION

This application claims priority to U.S. Ser. No. 61/366,281, filed onJul. 21, 2010.

FIELD OF THE INVENTION

The subject invention generally relates to a proppant and a method offorming the proppant. More specifically, the subject invention relatesto a proppant which comprises a particle and a coating disposed on theparticle, and which is used during hydraulic fracturing of asubterranean formation.

DESCRIPTION OF THE RELATED ART

Domestic energy needs in the United States currently outpace readilyaccessible energy resources, which has forced an increasing dependenceon foreign petroleum fuels, such as oil and gas. At the same time,existing United States energy resources are significantly underutilized,in part due to inefficient oil and gas procurement methods and adeterioration in the quality of raw materials such as unrefinedpetroleum fuels.

Petroleum fuels are typically procured from subsurface reservoirs via awellbore. Petroleum fuels are currently procured from low-permeabilityreservoirs through hydraulic fracturing of subterranean formations, suchas bodies of rock having varying degrees of porosity and permeability.Hydraulic fracturing enhances production by creating fractures thatemanate from the subsurface reservoir or wellbore, and providesincreased flow channels for petroleum fuels. During hydraulicfracturing, specially-engineered carrier fluids are pumped at highpressure and velocity into the subsurface reservoir to cause fracturesin the subterranean formations. A propping agent, i.e., a proppant, ismixed with the carrier fluids to keep the fractures open when hydraulicfracturing is complete. The proppant typically comprises a particle anda coating disposed on the particle. The proppant remains in place in thefractures once the high pressure is removed, and thereby props open thefractures to enhance petroleum fuel flow into the wellbore.Consequently, the proppant increases procurement of petroleum fuel bycreating a high-permeability, supported channel through which thepetroleum fuel can flow.

However, many existing proppants exhibit inadequate thermal stabilityfor high temperature and pressure applications, e.g. wellbores andsubsurface reservoirs having temperatures greater than 70° F. andpressures, i.e., closure stresses, greater than 7,500 psi. As an exampleof a high temperature application, certain wellbores and subsurfacereservoirs throughout the world have temperatures of about 375° F. and540° F. As an example of a high pressure application, certain wellboresand subsurface reservoirs throughout the world have closure stressesthat exceed 12,000 or even 14,000 psi. As such, many existing proppants,which comprise coatings, such as epoxy or phenolic coatings, which melt,degrade, and/or shear off the particle in an uncontrolled manner whenexposed to such high temperatures and pressures. Also, many existingproppants do not include active agents, such as microorganisms andcatalysts, to improve the quality of the petroleum fuel recovered fromthe subsurface reservoir.

Further, many existing proppants comprise coatings having inadequatecrush resistance. That is, many existing proppants comprise non-uniformcoatings that include defects, such as gaps or indentations, whichcontribute to premature breakdown and/or failure of the coating. Sincethe coating typically provides a cushioning effect for the proppant andevenly distributes high pressures around the proppant, prematurebreakdown and/or failure of the coating undermines the crush resistanceof the proppant. Crushed proppants cannot effectively prop openfractures and often contribute to impurities in unrefined petroleumfuels in the form of dust particles.

Moreover, many existing proppants also exhibit unpredictableconsolidation patterns and suffer from inadequate permeability inwellbores, i.e., the extent to which the proppant allows the flow ofpetroleum fuels. That is, many existing proppants have a lowerpermeability and impede petroleum fuel flow. Further, many existingproppants consolidate into aggregated, near-solid, non-permeableproppant packs and prevent adequate flow and procurement of petroleumfuels from subsurface reservoirs.

Also, many existing proppants are not compatible with low-viscositycarrier fluids having viscosities of less than about 3,000 cps at 80° C.Low-viscosity carrier fluids are typically pumped into wellbores athigher pressures than high-viscosity carrier fluids to ensure properfracturing of the subterranean formation. Consequently, many existingcoatings fail mechanically, i.e., shear off the particle, when exposedto high pressures or react chemically with low-viscosity carrier fluidsand degrade.

Finally, many existing proppants are coated via noneconomical coatingprocesses and therefore contribute to increased production costs. Thatis, many existing proppants require multiple layers of coatings, whichresults in time-consuming and expensive coating processes.

Due to the inadequacies of existing proppants, there remains anopportunity to provide an improved proppant.

SUMMARY OF THE INVENTION AND ADVANTAGES

The subject invention provides a proppant for hydraulically fracturing asubterranean formation. The subject invention also provides a method offorming the proppant comprising a particle and a polyamide imide coatingdisposed on the particle. The method comprises the steps of providingthe particle, providing the polyamide imide coating, and coating theparticle with the polyamide imide coating. A method of hydraulicallyfracturing a subterranean formation is also provided.

Advantageously, the proppant of the subject invention improves upon theperformance of existing proppants. The performance of the proppant isattributable to the polyamide imide coating. In addition, the proppantof the subject invention is formed efficiently, requiring few resources.

DETAILED DESCRIPTION OF THE INVENTION

The subject invention includes a proppant, a method of forming, orpreparing, the proppant, a method of hydraulically fracturing asubterranean formation, and a method of filtering a fluid. The proppantis typically used, in conjunction with a carrier fluid, to hydraulicallyfracture the subterranean formation which defines a subsurface reservoir(e.g. a wellbore or reservoir itself). Here, the proppant props open thefractures in the subterranean formation after the hydraulic fracturing.In one embodiment, the proppant may also be used to filter unrefinedpetroleum fuels, e.g. crude oil, in fractures to improve feedstockquality for refineries. However, it is to be appreciated that theproppant of the subject invention can also have applications beyondhydraulic fracturing and crude oil filtration, including, but notlimited to, water filtration and artificial turf.

The proppant comprises a particle and a polyamide imide coating disposedon the particle. As used herein, the terminology “disposed on”encompasses the polyamide imide coating being disposed about theparticle and also encompasses both partial and complete covering of theparticle by the polyamide imide coating. The polyamide imide coating isdisposed on the particle to an extent sufficient to change theproperties of the particle, e.g., to form a particle having a polyamideimide coating thereon which can be effectively used as a proppant. Assuch, any given sample of the proppant typically includes particleshaving the polyamide imide coating disposed theron, and the polyamideimide coating is typically disposed on a large enough surface area ofeach individual particle so that the sample of the proppant caneffectively prop open fractures in the subterranean formation during andafter the hydraulic fracturing, filter crude oil, etc. The polyamideimide coating is described additionally below.

Although the particle may be of any size, the particle typically has aparticle size distribution of from 10 to 100 mesh, more typically 20 to70 mesh, as measured in accordance with standard sizing techniques usingthe United States Sieve Series. That is, the particle typically has aparticle size of from 149 to 2,000, more typically of from 210 to 841,μm. Particles having such particle sizes allow less polyamide imidecoating to be used, allow the polyamide imide coating to be applied tothe particle at a lower viscosity, and allow the polyamide imide coatingto be disposed on the particle with increased uniformity andcompleteness as compared to particles having other particle sizes.

Although the shape of the particle is not critical, particles having aspherical shape typically impart a smaller increase in viscosity to ahydraulic fracturing composition than particles having other shapes, asset forth in more detail below. The hydraulic fracturing composition isa mixture comprising the carrier fluid and the proppant. Typically, theparticle is either round or roughly spherical.

The particle typically contains less than 1 part by weight of moisture,based on 100 parts by weight of the particle. Particles containinghigher than 1 part by weight of moisture typically interfere with sizingtechniques and prevent uniform polyamide imide coating of the particle.

Suitable particles for purposes of the subject invention include anyknown particle for use during hydraulic fracturing, water filtration, orartificial turf preparation. Non-limiting examples of suitable particlesinclude minerals, ceramics such as sintered ceramic particles, sands,nut shells, gravels, mine tailings, coal ashes, rocks, smelter slag,diatomaceous earth, crushed charcoals, micas, sawdust, wood chips,resinous particles, polymeric particles, and combinations thereof. It isto be appreciated that other particles not recited herein may also besuitable for the purposes of the subject invention.

Sand is a preferred particle and when applied in this technology iscommonly referred to as frac, or fracturing, sand. Examples of suitablesands include, but are not limited to, Arizona sand, Wisconsin sand,Badger sand, Brady sand, and Ottawa sand. Based on cost andavailability, inorganic materials such as sand and sintered ceramicparticles are typically favored for applications not requiringfiltration.

A specific example of a sand that is suitable as a particle for thepurposes of the subject invention is Arizona sand. Arizona sand iscommercially available from BASF Corporation of Florham Park, N.J.Arizona sand is a natural grain that is derived from weathering anderosion of preexisting rocks. As such, this sand is typically coarse andis roughly spherical. Another specific example of a sand that issuitable as a particle for the purposes of this invention is Ottawasand, commercially available from U.S. Silica Company of BerkeleySprings, W. Va. Yet another specific example of a sand that is suitableas a particle for the purposes of this invention is Badger sand,commercially available from Badger Mining Corporation of Berlin, Wis.Particularly preferred sands for application in this invention areOttawa and Badger sands. Ottawa and Badger sands of various sizes, suchas 30/50, 20/40, 40/70, and 140/70 can be used.

Specific examples of suitable sintered ceramic particles include, butare not limited to, aluminum oxide, silica, bauxite, and combinationsthereof. The sintered ceramic particle may also include clay-likebinders.

An active agent may also be included in the particle. In this context,suitable active agents include, but are not limited to, organiccompounds, microorganisms, and catalysts. Specific examples ofmicroorganisms include, but are not limited to, anaerobicmicroorganisms, aerobic microorganisms, and combinations thereof. Asuitable microorganism for the purposes of the subject invention iscommercially available from LUCA Technologies of Golden, Colo. Specificexamples of suitable catalysts include fluid catalytic crackingcatalysts, hydroprocessing catalysts, and combinations thereof. Fluidcatalytic cracking catalysts are typically selected for applicationsrequiring petroleum gas and/or gasoline production from crude oil.Hydroprocessing catalysts are typically selected for applicationsrequiring gasoline and/or kerosene production from crude oil. It is alsoto be appreciated that other catalysts, organic or inorganic, notrecited herein may also be suitable for the purposes of the subjectinvention.

Such additional active agents are typically favored for applicationsrequiring filtration. As one example, sands and sintered ceramicparticles are typically useful as a particle for support and proppingopen fractures in the subterranean formation which defines thesubsurface reservoir, and, as an active agent, microorganisms andcatalysts are typically useful for removing impurities from crude oil orwater. Therefore, a combination of sands/sintered ceramic particles andmicroorganisms/catalysts as active agents are particularly preferred forcrude oil or water filtration.

Suitable particles for purposes of the present invention may even beformed from resins and polymers. Specific examples of resins andpolymers for the particle include, but are not limited to,polyurethanes, polycarbodiimides, polyureas, acrylics,polyvinylpyrrolidones, acrrylonitrile-butadiene styrenes, polystyrenes,polyvinyl chlorides, fluoroplastics, polysulfides, nylon, polyamideimides and combinations thereof.

As indicated above, the proppant includes the polyamide imide coatingdisposed on the particle. The polyamide imide coating is selected basedon the desired properties and expected operating conditions of theproppant. The polyamide imide coating may provide the particle withprotection from operating temperatures and pressures in the subterraneanformation and/or subsurface reservoir. Further, the polyamide imidecoating may protect the particle against closure stresses exerted by thesubterranean formation. The polyamide imide coating may also protect theparticle from ambient conditions and minimizes disintegration and/ordusting of the particle. In some embodiments, the polyamide imidecoating may also provide the proppant with desired chemical reactivityand/or filtration capability.

The polyamide imide coating comprises the reaction product of anisocyanate and an isocyanate-reactive component. More specifically, thepolyamide amide coating typically comprises the reaction product of theisocyanate and the isocyanate-reactive component that comprises acarboxylic acid anhydride, as described additionally below. Theisocyanate is typically selected such that the physical properties ofthe polyamide imide coating, such as hardness, strength, toughness,creep, and brittleness are optimized.

The isocyanate may be any type of isocyanate known to those skilled inthe art. The isocyanate may be a polyisocyanate having two or morefunctional groups, e.g. two or more NCO functional groups. Suitableisocyanates for purposes of the present invention include, but are notlimited to, aliphatic and aromatic isocyanates. In various embodiments,the isocyanate is selected from the group of diphenylmethanediisocyanates (MDIs), polymeric diphenylmethane diisocyanates (pMDIs),toluene diisocyanates (TDIs), hexamethylene diisocyanates (HDIs),isophorone diisocyanates (IPDIs), and combinations thereof.

The isocyanate may be an isocyanate prepolymer. The isocyanateprepolymer is typically a reaction product of an isocyanate and a polyoland/or a polyamine. The isocyanate used in the prepolymer can be anyisocyanate as described above. The polyol used to form the prepolymer istypically selected from the group of ethylene glycol, diethylene glycol,propylene glycol, dipropylene glycol, butane diol, glycerol,trimethylolpropane, triethanolamine, pentaerythritol, sorbitol,biopolyols, and combinations thereof. The polyamine used to form theprepolymer is typically selected from the group of ethylene diamine,toluene diamine, diaminodiphenylmethane and polymethylene polyphenylenepolyamines, aminoalcohols, and combinations thereof. Examples ofsuitable aminoalcohols include ethanolamine, diethanolamine,triethanolamine, and combinations thereof.

Specific isocyanates that may be used to prepare the polyamide imidecoating include, but are not limited to, toluylene diisocyanate;4,4′-diphenylmethane diisocyanate; m-phenylene diisocyanate;1,5-naphthalene diisocyanate; 4-chloro-1; 3-phenylene diisocyanate;tetramethylene diisocyanate; hexamethylene diisocyanate;1,4-dicyclohexyl diisocyanate; 1,4-cyclohexyl diisocyanate,2,4,6-toluylene triisocyanate,1,3-diisopropylphenylene-2,4-diisocyanate;1-methyl-3,5-diethylphenylene-2,4-diisocyanate;1,3,5-triethylphenylene-2,4-diisocyanate;1,3,5-triisoproply-phenylene-2,4-diisocyanate;3,3′-diethyl-bisphenyl-4,4′-diisocyanate;3,5,3′,5′-tetraethyl-diphenylmethane-4,4′-diisocyanate;3,5,3′,5′-tetraisopropyldiphenylmethane-4,4′-diisocyanate;1-ethyl-4-ethoxy-phenyl-2,5-diisocyanate; 1,3,5-triethylbenzene-2,4,6-triisocyanate; 1-ethyl-3,5-diisopropylbenzene-2,4,6-triisocyanate and 1,3,5-triisopropylbenzene-2,4,6-triisocyanate. Other suitable polyamide imide coatings canalso be prepared from aromatic diisocyanates or isocyanates having oneor two aryl, alkyl, arakyl or alkoxy substituents wherein at least oneof these substituents has at least two carbon atoms. Specific examplesof suitable isocyanates include LUPRANATE® L5120, LUPRANATE® M,LUPRANATE® ME, LUPRANATE® MI, LUPRANATE® M20, and LUPRANATE® M70, allcommercially available from BASF Corporation of Florham Park, N.J.

In a particularly preferred embodiment, the isocyanate is a polymericisocyanate, such as LUPRANATE® M20. LUPRANATE® M20 comprises polymericdiphenylmethane diisocyanate and has an NCO content of about 31.5 weightpercent.

Typically, the amount of isocyanate which is reacted with theisocyanate-reactive component to form the polyamide imide coating isfrom about 0.01 to about 14, typically from about 0.05 to about 10 andmore typically from about 0.1 to about 3.5, percent by weight, based on100 parts by weight of the proppant. Of course, the amount of isocyanatewhich is reacted with the isocyanate-reactive component to form thepolyamide imide coating may vary outside of the ranges above, but istypically both whole and fractional values within about 0.01 to about 14percent by weight, based on 100 parts by weight of the proppant.

As described above, the isocyanate is reacted with theisocyanate-reactive component. Typically, the isocyanate-reactivecomponent comprises the carboxylic acid anhydride. Theisocyanate-reactive component can comprise one or more carboxylic acidanhydrides. In a preferred embodiment the isocyanate-reactive componentcomprises an aromatic carboxylic acid anhydride. Suitable aromaticcarboxylic acid anhydrides include, but are not limited to, trimelliticanhydride and pyromellitic dianhydride. In one embodiment, the aromaticcarboxylic acid anhydride is pyromellitic dianhydride. In anotherembodiment, the aromatic carboxylic acid anhydride is a mixture ofpyromellitic dianhydride and trimellitic anhydride. Typically, thearomatic carboxylic acid anhydride is trimellitic anhydride. Thecarboxylic acid anhydride may be reacted, to form the polyamide imidecoating, in an amount of from about 0.06 to about 14, typically fromabout 0.08 to about 10, and more typically from about 0.1 to about 8,percent by weight, based on 100 parts by weight of the proppant. Ofcourse, the amount of carboxylic acid anhydride reacted with theisocyanate to form the polyamide imide coating may vary outside of theranges above, but is typically both whole and fractional values withinabout 0.06 to about 14 percent by weight, based on 100 parts by weightof the proppant.

The isocyanate-reactive component can comprise the reaction product ofthe carboxylic acid anhydride and an amine. Typically, the amine isselected from the group of polyether amines, amino silanes, andcombinations thereof. An amino functional group of the amine typicallyreacts with an anhydride functional group of the carboxylic acidanhydride to form an imide linkage. The amine is commonly referred to inthe art as a compatibilizer, a flexiblizer, an adhesion promoter, acoupling agent, and/or a resin additive to improve various performanceproperties of the polyamide imide coating, such as reduction ofbrittleness. Typically, the amine is reacted, to form the polyamideimide coating, in an amount of from about 0.001 to about 5, typicallyfrom about 0.01 to about 2, and more typically from about 0.05 to about1, percent by weight, based on 100 parts by weight of the proppant. Ofcourse, the amount of amine reacted to form the polyamide imide coatingmay vary outside of the ranges above, but is typically both whole andfractional values within about 0.001 to about 5 percent by weight, basedon 100 parts by weight of the proppant.

The polyether amine of the present invention chemically reacts with thecarboxylic acid anhydride. More specifically, the amino functional groupof the polyether amine reacts with the anhydride functional group of thecarboxylic acid anhydride to form a reaction product having an imidelinkage as well as polyether functionality. In turn, the reactionproduct is further chemically reacted with the isocyanate to form thepolyamide imide coating having polyether functionality.

Typically, the polyether amine includes the amino functional groupattached to a polyether backbone. The polyether backbone is formed froman alkylene oxide such as ethylene oxide, propylene oxide, and/orbutylene oxide. The polyether amine can be a mono amine, a diamine,and/or a triamine. The amino functional group can be primary, secondary,or tertiary. A specific example of a suitable polyether amine isJEFFAMINE® D-400, i.e., polyoxypropylenediamine, commercially availablefrom Huntsman of The Woodlands, Tex. JEFFAMINE® D-400 is a diamine. Morespecifically, JEFFAMINE® D-400 is a polyether backbone having twoprimary amino functional groups bonded thereto. JEFFAMINE® D-400 has amolecular weight of about 430.

The amino silane of the present invention chemically reacts with thecarboxylic acid anhydride. More specifically the amino functional groupof the amino silane reacts with the anhydride functional group of thecarboxylic acid anhydride to form a reaction product having an imidelinkage and silane functionality. In turn, the reaction product isfurther chemically reacted with the isocyanate to form the polyamideimide coating having silane functionality. The silane functionalityimproves the chemical bonding of the polyamide imide coating to theparticle.

Examples of suitable amino silanes include, but are not limited to,gamma-aminopropyltriethoxysilane, gamma-aminopropylsilsesquioxane,modified aminoorganosilane, gamma-aminopropyltrimethoxysilane,N-beta-(aminoethyl)-gamma-aminopropyltrimethoxysilane, triaminofunctional silane, bis-(gamma-trimethoxysilylpropyl)amine, polyazamidesilane, delta-aminoneohexyltrimethoxysilane,N-beta-(aminoethyl)-gamma-aminopropylmethyldimethoxysilane,delta-aminoneohexylmethyldimethoxysilane,N-phenyl-gamma-aminopropyltrimethoxysilane, and combinations thereof.Specific examples of suitable amino silanes include, but are not limitedto, Silquest™ A1100, Silquest™ 1106, Silquest™ A1108, Silquest™ A1110,Silquest™ A1120, Silquest™ A1130, Silquest™ A1170, Silquest™ A1387,Silquest™ A1637, Silquest™ A2120, Silquest™ A2639, and Silquest™ Y9669,all commercially available from Momentive Performance Materials ofAlbany, N.Y. A particularly suitable amino silane is Silquest™ A1100,i.e., gamma-aminopropyltriethoxysilane. Another particularly suitableamino silane is Silquest™ A1120, i.e.,N-beta-(aminoethyl)-gamma-aminopropyltrimethoxysilane.

The isocyanate-reactive component may also include a catalyst. Thecatalyst may be used to catalyze the reaction between the carboxylicacid anhydride, the isocyanate, and/or the amine. For example, thecatalyst may be used to reduce the temperature at which the carboxylicacid anhydride and the isocyanate chemically react. Theisocyanate-reactive component may optionally include more than onecatalyst. The catalyst may include any suitable catalyst or mixtures ofcatalysts known in the art. Suitable catalysts for purposes of thepresent invention typically include catalytic amines, such as primary,secondary, and tertiary, cyclic and acyclic catalytic amines. A specificexample of a suitable catalyst is triethylamine, a tertiary amine,commercially available from BASF Corporation of Florham Park, N.J. Thecatalyst may be present in the proppant in any amount sufficient tocatalyze the reaction between the carboxylic acid anhydride, theisocyanate, and/or the amine. The catalyst is typically present in theproppant in an amount of from about 0.0001 to about 10, more typicallyfrom about 0.0015 to about 5, and most typically from about 0.001 toabout 2, percent by weight, based on 100 parts by weight of theproppant. Of course, the amount of catalyst present in the proppant mayvary outside of the ranges above, but is typically both whole andfractional values within about 0.0001 to about 10 percent by weight,based on 100 parts by weight of the proppant.

The isocyanate-reactive component may further include additives.Suitable additives include, but are not limited to, surfactants, blowingagents, wetting agents, blocking agents, dyes, pigments, diluents,solvents, specialized functional additives such as antioxidants,ultraviolet stabilizers, biocides, adhesion promoters, antistaticagents, fire retardants, fragrances, and combinations of the group. Forexample, a pigment allows the polyamide imide coating to be visuallyevaluated for thickness and integrity and can provide various marketingadvantages. Also, physical blowing agents and chemical blowing agentsare typically selected for polyamide imide coatings requiring foaming.That is, in one embodiment, the coating may comprise a foam coatingdisposed on the particle. Again, it is to be understood that theterminology “disposed on” encompasses both partial and complete coveringof the particle by the polyamide imide coating, a foam coating in thisinstance. The foam coating is typically useful for applicationsrequiring enhanced contact between the proppant and crude oil. That is,the foam coating typically defines microchannels and increases a surfacearea for contact between crude oil and the catalyst and/ormicroorganism.

As described above, the polyamide imide coating comprises the reactionproduct of the isocyanate and the isocyanate-reactive componentcomprising the carboxylic acid anhydride. More specifically, thepolyamide imide coating is a polymer comprising amide linkages and imidelinkages. The amide linkages are typically formed from a chemicalreaction of the isocyanate and a carboxylic acid functional group of thecarboxylic acid anhydride. The imide linkages are typically formed froma chemical reaction of the isocyanate and an anhydride functional groupof the carboxylic acid anhydride. However, it is to be understood thatthe amide linkages and the imide linkages of the polyamide imide coatingcan be formed from other reactants and other chemical reactions.Typically, the polyamide imide coating has, as the name suggests, apositive synergy of properties from both the amide and imide linkages,such as durability, strength, melt processability, exceptional high heatcapability, and broad chemical resistance.

The polyamide imide coating is typically selected for applicationsrequiring excellent coating stability and adhesion to the particle.Further, polyamide imide coating is typically selected based on thedesired properties and expected operating conditions of a particularapplication. As one example, the polyamide imide coating is particularlyapplicable when the proppant is exposed to significant compressionand/or shear forces, and temperatures exceeding 500° C. in thesubterranean formation and/or subsurface reservoir defined by theformation. The polyamide imide coating is generally viscous to solidnature, and depending on molecular weight. Any suitable polyamide imidecoating may be used for the purposes of the subject invention. Thepolyamide imide coating is typically present in the proppant in anamount of from about 0.1 to about 20, more typically of from about 0.5to about 7.5, and most typically of from about 1.0 to about 6.0, percentby weight based on 100 parts by weight of the particle. Of course, theamount of polyamide imide coating present in the proppant may varyoutside of the ranges above, but is typically both whole and fractionalvalues within about 0.1 to about 20 percent by weight, based on 100parts by weight of the particle.

The polyamide imide coating may be formed in-situ where the polyamideimide coating is disposed on the particle during formation of thepolyamide imide coating. Said differently, the components of thepolyamide imide coating are typically combined with the particle and thepolyamide imide coating is disposed on the particle.

However, in one embodiment a polyamide imide coating is formed and sometime later applied to, e.g. mixed with, the particle and exposed totemperatures exceeding 100° C. to coat the particle and form theproppant. Advantageously, this embodiment allows the polyamide imidecoating to be formed at a location designed to handle chemicals, underthe control of personnel experienced in handling chemicals. Once formed,the polyamide imide coating can be transported to another location,applied to the particle, and heated. There are numerous logistical andpractical advantages associated with this embodiment. For example, ifthe polyamide imide coating is being applied to the particle, e.g. fracsand, the polyamide imide coating may be applied immediately followingthe manufacturing of the frac sand, when the frac sand is already atelevated temperature, eliminating the need to reheat the polyamide imidecoating and the frac sand, thereby reducing the amount of energyrequired to form the proppant.

In another embodiment, the isocyanate-reactive component, such astrimellitic anhydride, and the isocyanate, such as polymeric isocyanate,are reacted to form the polyamide imide coating in a solution. Thesolution comprises a solvent such as acetone. The solution viscosity iscontrolled by stoichiometry, monofunctional reagents, and a polymersolids level. After the polyamide imide is formed in the solution, thesolution is applied to the particle. The solvent evaporates leaving thepolyamide imide coating disposed on the particle. Once the polyamideimide coating is disposed on the particle to form the proppant, theproppant can be heated to further crosslink the polyamide imide coating.Generally, the crosslinking, which occurs as a result of the heating,optimizes physical properties of the polyamide imide coating.

The polyamide imide coating may also be further defined ascontrolled-release. That is, the polyamide imide coating maysystematically dissolve, hydrolyze in a controlled manner, or physicallyexpose the particle to the petroleum fuels in the subsurface reservoir.The polyamide imide coating typically gradually dissolves in aconsistent manner over a pre-determined time period to decrease thethickness of the polyamide imide coating. This embodiment is especiallyuseful for applications utilizing the active agent such as themicroorganism and/or the catalyst. That is, the polyamide imide coatingis typically controlled-release for applications requiring filtration ofpetroleum fuels or water.

The polyamide imide coating may exhibit excellent non-wettability in thepresence of water, as measured in accordance with standard contact anglemeasurement methods known in the art. The polyamide imide coating mayhave a contact angle of greater than 90° and may be categorized ashydrophobic. Consequently, the proppant of such an embodiment canpartially float in the subsurface reservoir and is typically useful forapplications requiring foam coatings.

The polyamide imide coating of the present invention can be crosslinkedwhere it is cured prior to pumping of the proppant into the subsurfacereservoir, or the polyamide imide coating can be curable whereby thepolyamide imide coating cures in the subsurface reservoir due to theconditions inherent therein. These concepts are described further below.

The proppant of the subject invention may comprise the particleencapsulated with a crosslinked polyamide imide coating. The crosslinkedpolyamide imide coating typically provides crush strength, orresistance, for the proppant and prevents agglomeration of the proppant.Since the crosslinked polyamide imide coating is cured before theproppant is pumped into a subsurface reservoir, the proppant typicallydoes not crush or agglomerate even under high pressure and temperatureconditions.

Alternatively, the proppant of the subject invention may comprise theparticle encapsulated with a curable polyamide imide coating. Thecurable polyamide imide coating typically consolidates and curessubsurface. The curable polyamide imide coating is typically notcrosslinked, i.e., cured, or partially crosslinked before the proppantis pumped into the subsurface reservoir. Instead, the curable polyamideimide coating typically cures under the high pressure and temperatureconditions in the subsurface reservoir. Proppants comprising theparticle encapsulated with the curable polyamide imide coating are oftenused for high pressure and temperature conditions.

Additionally, proppants comprising the particle encapsulated with thecurable polyamide imide coating may be classified as curable proppants,subsurface-curable proppants and partially-curable proppants.Subsurface-curable proppants typically cure entirely in the subsurfacereservoir, while partially-curable proppants are typically partiallycured before being pumped into the subsurface reservoir. Thepartially-curable proppants then typically fully cure in the subsurfacereservoir. The proppant of the subject invention can be eithersubsurface-curable or partially-curable.

Multiple layers of the polyamide imide coating can be applied to theparticle to form the proppant. As such, the proppant of the subjectinvention can comprise a particle having a crosslinked polyamide imidecoating disposed on the particle and a curable polyamide imide coatingdisposed on the crosslinked coating, and vice versa. Likewise, multiplelayers of the polyamide imide coating, each individual layer having thesame or different physical properties, can be applied to the particle toform the proppant. In addition, the polyamide imide coating can beapplied to the particle in combination with coatings of differentmaterials such as polyurethane coatings, polycarbodiimide coatings, andother material coatings.

The proppant may further include a silicon-containing adhesion promoter.This adhesion promoter is also commonly referred to in the art as acoupling agent or as a binder agent. The adhesion promoter binds thepolyamide imide coating to the particle. More specifically, the adhesionpromoter typically has organofunctional silane groups to improveadhesion of the polyamide imide coating to the particle. Without beingbound by theory, it is thought that the adhesion promoter allows forcovalent bonding between the particle and the polyamide imide coating.In one embodiment, the surface of the particle is activated with theadhesion promoter by applying the adhesion promoter to the particleprior to coating the particle with the polyamide imide coating. In thisembodiment, the adhesion promoter can be applied to the particle by awide variety of application techniques including, but not limited to,spraying, dipping the particles in the polyamide imide coating, etc. Inanother embodiment, the adhesion promoter may be added to theisocyanate-reactive component. As such, the particle is then simplyexposed to the adhesion promoter when the polyamide imide coating isapplied to the particle. The adhesion promoter is useful forapplications requiring excellent adhesion of the polyamide imide coatingto the particle, for example, in applications where the proppant issubjected to shear forces in an aqueous environment. Use of the adhesionpromoter provides adhesion of the polyamide imide coating to theparticle such that the polyamide imide coating will remain adhered tothe surface of the particle even if the proppant, including thepolyamide imide coating, the particle, or both, fractures due to closurestress.

Examples of suitable adhesion promoters, which are silicon-containing,include, but are not limited to, glycidoxypropyltrimethoxysilane,aminoethylaminopropyltrimethoxysilane,methacryloxypropyltrimethoxysilane, gamma-aminopropyltriethoxysilane,vinylbenzylaminoethylaminopropyltrimethoxysilane,glycidoxypropylmethyldiethoxysilane, chloropropyltrimethoxysilane,phenyltrimethoxysilane, vinyltriethoxysilane, tetraethoxysilane,methyldimethoxysilane, bis-triethoxysilylpropyldisulfidosilane,bis-triethoxysilylpropyltetrasulfidosilane, phenyltriethoxysilane,aminosilanes, and combinations thereof.

Specific examples of suitable adhesion promoters include, but are notlimited to, Silquest™ A1100, Silquest™ A1110, Silquest™ A1120, Silquest™1130, Silquest™ A1170, Silquest™ A-189, and Silquest™ Y9669, allcommercially available from Momentive Performance Materials of Albany,N.Y. A particularly suitable silicon-containing adhesion promoter isSilquest™ A1100, i.e., gamma-aminopropyltriethoxysilane. Thesilicon-containing adhesion promoter may be present in the proppant inan amount of from about 0.001 to about 10, typically from about 0.01 toabout 5, and more typically from about 0.05 to about 2, percent byweight, based on 100 parts by weight of the proppant. Of course, theamount of silicon-containing adhesion promoter present in the proppantmay vary outside of the ranges above, but is typically both whole andfractional values within about 0.001 to about 10 percent by weight,based on 100 parts by weight of the proppant.

The proppant may further include a wetting agent. The wetting agent isalso commonly referred to in the art as a surfactant. The proppant mayinclude more than one wetting agent. The wetting agent may include anysuitable wetting agent or mixtures of wetting agents know in the art.The wetting agent is employed to increase a surface area contact betweenthe polyamide imide coating and the particle. In a typical embodiment,the wetting agent is added to the isocyanate-reactive component. Inanother embodiment, the surface of the particle is activated with thewetting agent by applying the wetting agent to the particle prior tocoating the particle with the polyamide imide coating.

A suitable wetting agent is BYK® 310, a polyester modifiedpoly-dimethyl-siloxane, commercially available from BYK Additives andInstruments of Wallingford, Conn. The wetting agent may be present inthe proppant in an amount of from about 0.001 to about 10, typicallyfrom about 0.002 to about 5, and more typically from about 0.004 toabout 2, percent by weight, based on 100 parts by weight of theproppant. Of course, the amount of wetting agent present in the proppantmay vary outside of the ranges above, but is typically both whole andfractional values within about 0.001 to about 10 percent by weight,based on 100 parts by weight of the proppant.

The polyamide imide coating of this invention may also include theactive agent already described above in the context of the particle. Inother words, the active agent may be included in the polyamide imidecoating independent of the particle. Once again, suitable active agentsinclude, but are not limited to organic compounds, microorganisms, andcatalysts. The polyamide imide coating may include other additives,active or otherwise, such as wetting agents, surfactants, and the like.

The proppant of the subject invention typically exhibits excellentthermal stability for high temperature and pressure applications, e.g.temperatures greater than 100° C., typically greater than 250° C., moretypically greater than 350° C., and even more typically greater than500° C., and/or pressures (independent of the temperatures describedabove) greater than 7,500 psi, typically greater than 10,000 psi, moretypically greater than 12,500 psi, and even more typically greater than15,000 psi. The proppant of this invention does not suffer from completefailure of the polyamide imide coating due to shear or degradation whenexposed to such temperatures and pressures.

Further, with the polyamide imide coating of this invention, theproppant typically exhibits excellent crush strength, also commonlyreferred to as crush resistance. With this crush strength, the polyamideimide coating of the proppant is uniform and is substantially free fromdefects, such as gaps or indentations, which often contribute topremature breakdown and/or failure of the polyamide imide coating. Inparticular, the proppant exhibits a crush strength of 15% or lessmaximum fines as measured in accordance with American PetroleumInstitute (API) RP60 at pressures ranging from 7500 to 15000 psi,including at specific stress pressures of 7500, 10000, 12500, and 15000psi.

When 20/40 Arizona sand is utilized as the particle, a preferred crushstrength associated with the proppant of this invention is 10% or less,more preferred 5% or less, maximum fines as measured in accordance withAPI RP60 at the same stress pressure range and specific stress pressuresdescribed above. When 40/70 Ottawa sand is utilized as the particle, acrush strength associated with the proppant of this invention istypically less than 8%, more typically 5%, and most typically 2% maximumfines as measured in accordance with API RP60 at the same stresspressure range and specific stress pressures described above. In oneembodiment where 40/70 Ottawa sand is utilized as the particle, thecrush strength of this proppant is 1.5% fines as measured in accordancewith API RP60 at 7500 psi.

In addition to testing crush strength in accordance with the parametersset forth in API RP60, the crush strength of the proppant can be testedwith various other testing parameters. For example, a sample of theproppant can be sieved to a sieve size of greater than 35. Once sievedand tested, the proppant of the present invention typically has a crushstrength of 15%, more typically 10%, even more typically 5%, and mosttypically 2.5%, or less maximum fines less than sieve size 35 asmeasured by compressing a 9.4 g sample of the proppant in a testcylinder having a diameter of 1.5 inches for 1 hour at 7500 psi and 93°C.

The polyamide imide coating of this invention typically provides acushioning effect for the proppant and evenly distributes highpressures, e.g. closure stresses, around the proppant. Therefore, theproppant of the subject invention effectively props open fractures andminimizes unwanted impurities in unrefined petroleum fuels in the formof dust particles.

Although customizable according to carrier fluid selection, the proppanttypically has a specific gravity of from 0.1 to 3.0, more typically from1.0 to 2.0. One skilled in the art typically selects the specificgravity of the proppant according to the specific gravity of the carrierfluid and whether it is desired that the proppant be lightweight orsubstantially neutrally buoyant in the selected carrier fluid. Inparticular, it is desired that the specific gravity of the proppant isless than the specific gravity of the carrier fluid to minimize proppantsettling in the carrier fluid. Further, based on the non-wettability ofthe polyamide imide coating including crosslinks as set forth above, theproppant of such an embodiment typically has an apparent density, i.e.,a mass per unit volume of bulk proppant, also known as bulk density, offrom 2.0 to 3.0, more typically from 2.3 to 2.7, g/cm³ according to APIRecommended Practices RP60 for testing proppants. It is believed thatthe non-wettability of the polyamide imide coating may contribute toflotation of the proppant depending on the selection of the carrierfluid in the wellbore.

Further, the proppant typically minimizes unpredictable consolidation.That is, the proppant only consolidates, if at all, in a predictable,desired manner according to carrier fluid selection and operatingtemperatures and pressures. Also, the proppant is typically compatiblewith low-viscosity carrier fluids having viscosities of less than about3,000 cps at 80° C. and is typically substantially free from mechanicalfailure and/or chemical degradation when exposed to the carrier fluidsand high pressures. Finally, the proppant is typically coated viaeconomical coating processes and typically does not require multiplecoating layers, and therefore minimizes production costs.

As set forth above, the subject invention also provides the method offorming, or preparing, the proppant. For this method, the particle andthe polyamide imide coating are provided, and the particle is coatedwith the polyamide imide coating. The step of coating the particle withthe polyamide imide coating is described additionally below.

To provide the polyamide imide coating, the isocyanate is reacted withthe isocyanate-reactive component. As indicated in certain embodimentsbelow, the isocyanate may be reacted to form the polyamide imide coatingprior to the actual coating of the particle; alternatively, theisocyanate may be reacted to form the polyamide imide coatingsimultaneous with the actual coating of the particle.

As with all other components which may be used in the method of thesubject invention (e.g. the particle), the isocyanate and the isocyanatereactive component are just as described above with respect to thepolyamide imide coating.

The particle is coated with the polyamide imide coating to form theproppant. The polyamide imide coating is applied to the particle to coatthe particle. The particle may optionally be heated to a temperaturegreater than 150° C. prior to or simultaneous with the step of coatingthe particle with the polyamide imide coating. A preferred temperaturerange for heating the particle is 150-180° C. Heating the particlebrings the temperature of the particle closer to a temperature at whichthe components can react to form the polyamide imide coating or furthercross-link the polyamide imide coating.

Various techniques can be used to coat the particle with the polyamideimide coating. These techniques include, but are not limited to, mixing,pan coating, fluidized-bed coating, co-extrusion, spraying, in-situformation of the coating, and spinning disk encapsulation. The techniquefor applying the coating to the particle is selected according to cost,production efficiencies, and batch size.

In this method, the step of reacting the isocyanate in the presence ofthe catalyst to form the polyamide imide coating and the step of coatingthe particle with the polyamide imide coating are collectively conductedin 20 minutes or less, typically in 15 minutes or less, and moretypically in 1 to 10 minutes.

Once coated, the proppant can be heated to further crosslink thepolyamide imide coating. Generally, the crosslinking, which occurs as aresult of the heating, optimizes physical properties of the proppant,thereby optimizing the performance of the proppant. When the proppant isheated to further crosslink the polyamide imide coating, the proppant istypically heated to a temperature of from about 50 to about 500, moretypically from about 100 to about 400, and most typically from about 150to about 350° C. for any amount of time. In one embodiment, the proppantis heated to a temperature of about 265° C. for 1.25 hours. Where theproppant is heated to further crosslink the polyamide imide coating, thestep of reacting the isocyanate to form the polyamide imide coating, thestep of coating the particle with the polyamide imide coating, and ofthe step of heating the proppant to further crosslink the polyamideimide coating may be collectively conducted in 10 hours or less,typically in 1.5 hours or less, more typically in 1 hour or less, andmost typically in 0.5 hours or less.

In one embodiment, the polyamide imide coating is disposed on theparticle via mixing in a vessel, e.g. a reactor. In particular, theindividual components of the proppant, e.g. the isocyanate, theisocyanate-reactive component, and the particle, are added to the vesselto form a reaction mixture. The components may be added in equal orunequal weight ratios. The reaction mixture is typically agitated at anagitator speed commensurate with the viscosities of the components.Further, the reaction mixture is typically heated at a temperaturecommensurate with the polyamide imide coating technology and batch size.For example, the components of the polyamide imide coating are typicallyheated from a temperature of about 70° C. to a temperature of about 130°C. in 10 minutes or less, depending on batch size. It is to beappreciated that the technique of mixing may include adding componentsto the vessel sequentially or concurrently. Also, the components may beadded to the vessel at various time intervals and/or temperatures.

In another embodiment, the polyamide imide coating is disposed on theparticle via spraying. In particular, individual components of thepolyamide imide coating are contacted in a spray device to form acoating mixture. The coating mixture is then sprayed onto the particleto form the proppant. Spraying the polyamide imide coating onto theparticle results in a uniform, complete, and defect-free polyamide imidecoating disposed on the particle. For example, the polyamide imidecoating is typically even and unbroken. The polyamide imide coating alsotypically has adequate thickness and acceptable integrity, which allowsfor applications requiring controlled-release of the proppant in thefracture. Spraying also typically results in a thinner and moreconsistent polyamide imide coating disposed on the particle as comparedto other techniques, and thus the proppant is coated economically.Spraying the particle even permits a continuous manufacturing process.Spray temperature is typically selected by one known in the artaccording to polyamide imide coating technology and ambient humidityconditions. The particle may also be heated to induce crosslinking ofthe polyamide imide coating. Further, one skilled in the art typicallysprays the components of the polyamide imide coating at a viscositycommensurate with the viscosity of the components.

In another embodiment, the polyamide imide coating is disposed on theparticle in-situ, i.e., in a reaction mixture comprising the componentsof the polyamide imide coating and the particle. In this embodiment, thepolyamide imide coating is formed or partially formed as the polyamideimide coating is disposed on the particle. In-situ polyamide imidecoating formation steps typically include providing each component ofthe polyamide imide coating, providing the particle, combining thecomponents of the polyamide imide coating and the particle, anddisposing the polyamide imide coating on the particle. In-situ formationof the polyamide imide coating typically allows for reduced productioncosts by way of fewer processing steps as compared to existing methodsfor forming a proppant.

The formed proppant is typically prepared according to the method as setforth above and stored in an offsite location before being pumped intothe subterranean formation and the subsurface reservoir. As such,spraying typically occurs offsite from the subterranean formation andsubsurface reservoir. However, it is to be appreciated that the proppantmay also be prepared just prior to being pumped into the subterraneanformation and the subsurface reservoir. In this scenario, the proppantmay be prepared with a portable coating apparatus at an onsite locationof the subterranean formation and subsurface reservoir.

The proppant is useful for hydraulic fracturing of the subterraneanformation to enhance recovery of petroleum and the like. In a typicalhydraulic fracturing operation, a hydraulic fracturing composition,i.e., a mixture, comprising the carrier fluid, the proppant, andoptionally various other components, is prepared. The carrier fluid isselected according to wellbore conditions and is mixed with the proppantto form the mixture which is the hydraulic fracturing composition. Thecarrier fluid can be a wide variety of fluids including, but not limitedto, kerosene and water. Typically, the carrier fluid is water. Variousother components which can be added to the mixture include, but are notlimited to, guar, polysaccharides, and other components know to thoseskilled in the art.

The mixture is pumped into the subsurface reservoir, which may be thewellbore, to cause the subterranean formation to fracture. Morespecifically, hydraulic pressure is applied to introduce the hydraulicfracturing composition under pressure into the subsurface reservoir tocreate or enlarge fractures in the subterranean formation. When thehydraulic pressure is released, the proppant holds the fractures open,thereby enhancing the ability of the fractures to extract petroleumfuels or other fluids from the subsurface reservoir to the wellbore.

For the method of filtering a fluid, the proppant of the subjectinvention is provided according to the method of forming the proppant asset forth above. In one embodiment, the fluid can be an unrefinedpetroleum or the like. However, it is to be appreciated that the methodof the subject invention may include the filtering of other fluids notspecifically recited herein, for example, air, water, or natural gas.

To filter the fluid, the fracture in the subsurface reservoir thatcontains the unrefined petroleum, e.g. unfiltered crude oil, isidentified by methods known in the art of oil extraction. Unrefinedpetroleum is typically procured via a subsurface reservoir, such as awellbore, and provided as feedstock to refineries for production ofrefined products such as petroleum gas, naphtha, gasoline, kerosene, gasoil, lubricating oil, heavy gas, and coke. However, crude oil thatresides in subsurface reservoirs includes impurities such as sulfur,undesirable metal ions, tar, and high molecular weight hydrocarbons.Such impurities foul refinery equipment and lengthen refinery productioncycles, and it is desirable to minimize such impurities to preventbreakdown of refinery equipment, minimize downtime of refinery equipmentfor maintenance and cleaning, and maximize efficiency of refineryprocesses. Therefore, filtering is desirable.

For the method of filtering, the hydraulic fracturing composition ispumped into the subsurface reservoir so that the hydraulic fracturingcomposition contacts the unfiltered crude oil. The hydraulic fracturingcomposition is typically pumped into the subsurface reservoir at a rateand pressure such that one or more fractures are formed in thesubterranean formation. The pressure inside the fracture in thesubterranean formation may be greater than 5,000, greater than 7,000, oreven greater than 10,000 psi, and the temperature inside the fracture istypically greater than 70° F. and can be as high 375° F. depending onthe particular subterranean formation and/or subsurface reservoir.

Although not required for filtering, it is particularly desirable thatthe proppant be a controlled-release proppant. With a controlled-releaseproppant, while the hydraulic fracturing composition is inside thefracture, the polyamide imide coating of the proppant typicallydissolves in a controlled manner due to pressure, temperature, pHchange, and/or dissolution in the carrier fluid in a controlled manner,i.e., a controlled-release. Complete dissolution of the polyamide imidecoating depends on the thickness of the polyamide imide coating and thetemperature and pressure inside the fracture, but typically occurswithin 1 to 4 hours. It is to be understood that the terminology“complete dissolution” generally means that less than 1% of the coatingremains disposed on or about the particle. The controlled-release allowsa delayed exposure of the particle to crude oil in the fracture. In theembodiment where the particle includes the active agent, such as themicroorganism or catalyst, the particle typically has reactive sitesthat must contact the fluid, e.g. the crude oil, in a controlled mannerto filter or otherwise clean the fluid. If implemented, thecontrolled-release provides a gradual exposure of the reactive sites tothe crude oil to protect the active sites from saturation. Similarly,the active agent is typically sensitive to immediate contact with freeoxygen. The controlled-release provides the gradual exposure of theactive agent to the crude oil to protect the active agent fromsaturation by free oxygen, especially when the active agent is amicroorganism or catalyst.

To filter the fluid, the particle, which is substantially free of thepolyamide imide coating after the controlled-release, contacts thefluid, e.g. the crude oil. It is to be understood that the terminology“substantially free” means that complete dissolution of the polyamideimide coating has occurred and, as defined above, less than 1% of thepolyamide imide coating remains disposed on or about the particle. Thisterminology is commonly used interchangeably with the terminology“complete dissolution” as described above. In an embodiment where anactive agent is utilized, upon contact with the fluid, the particletypically filters impurities such as sulfur, unwanted metal ions, tar,and high molecular weight hydrocarbons from the crude oil throughbiological digestion. As noted above, a combination of sands/sinteredceramic particles and microorganisms/catalysts are particularly usefulfor filtering crude oil to provide adequate support/propping and also tofilter, i.e., to remove impurities. The proppant therefore typicallyfilters crude oil by allowing the delayed exposure of the particle tothe crude oil in the fracture.

The filtered crude oil is typically extracted from the subsurfacereservoir via the fracture, or fractures, in the subterranean formationthrough methods known in the art of oil extraction. The filtered crudeoil is typically provided to oil refineries as feedstock, and theparticle typically remains in the fracture.

Alternatively, in a fracture that is nearing its end-of-life, e.g. afracture that contains crude oil that cannot be economically extractedby current oil extraction methods, the particle may also be used toextract natural gas as the fluid from the fracture. The particle,particularly where an active agent is utilized, digests hydrocarbons bycontacting the reactive sites of the particle and/or of the active agentwith the fluid to convert the hydrocarbons in the fluid into propane ormethane. The propane or methane is then typically harvested from thefracture in the subsurface reservoir through methods known in the art ofnatural gas extraction.

The following examples are meant to illustrate the invention and are notto be viewed in any way as limiting to the scope of the invention.

EXAMPLES

Examples 1-5 are proppants formed according to the subject inventioncomprising the polyamide imide coating disposed on the particle.Examples 1-5 are formed with components disclosed in Table 1. Theamounts in Table 1 are in parts by weight, based on 100 parts by weightof the proppant.

TABLE 1 Component Example 1 Example 2 Example 3 Example 4 Example 5Carboxylic 0.907 0.907 0.955 0.955 0.963 Acid Anhydride Amine A 0.1600.160 — — — Amine B — — 0.072 0.072 — Amine C — — — — 0.073 Catalyst0.008 0.008 0.008 0.008 0.010 Isocyanate 1.429 1.429 1.469 1.469 1.460Adhesion 0.150 0.150 0.150 0.150 0.150 Promoter Wetting 0.006 0.0060.006 0.006 0.010 Agent Particle 97.340 97.340 97.340 97.340 97.334Total 100.000 100.000 100.000 100.000 100.000 Carboxylic Acid Anhydrideis trimellitic anhydride. Amine A is gamma-aminopropyltriethoxysilane.Amine B is N-beta-(aminoethyl)-gamma-aminopropyltrimethoxysilane. AmineC is polyoxypropylenediamine. Catalyst is triethylamine. Isocyanate is apolymeric polymeric diphenylmethane diisocyanate. Adhesion Promoter isgamma-aminopropyltriethoxysilane. Wetting Agent is polyester modifiedpoly-dimethyl-siloxane. Particle is Ottawa sand having a sieve size of20/40.

Prior to forming Examples 1-5, the Particle is activated with theAdhesion Promoter. The Particle, now activated, is added to a reactionvessel. The Carboxylic Acid Anhydride, the Catalyst, the Wetting Agent,and Amine A, B, and/or C, depending on the Example, and the Isocyanateare dissolved in acetone to form a solution. The solution is sprayedapplied onto the Particle in the reaction vessel to form a reactionmixture. The reaction mixture, at 265° C., is agitated to (1) uniformlycoat the surface of, or wet out, the Particle with the reaction mixtureand (2) polymerize the Carboxylic Acid Anhydride and the Isocyanate.Agitation continues for about 10 minutes. As such, Examples 1-5 areproppants comprising the Particle and the polyamide imide coating formedthereon. The proppants of Examples 1-5 are heated, i.e., post-cured tofurther cure the polyamide imide coating. Examples 1-5 are post-curedaccording to the parameters set forth below in Table 2.

TABLE 2 Post-Cure Exam- Exam- Exam- Exam- Exam- Parameter ple 1 ple 2ple 3 ple 4 ple 5 Temperature (° C.) 265 270 265 270 265 Time (hours) 13 1.25 3 1

Examples 1-5 are tested for crush strength, the test results are setforth in Table 3 below. The appropriate formula for determining percentfines is set forth in API RP60. Prior to testing crush strength,Examples 1-5 are sieved to ensure that a proppant sample comprisesindividual proppant particles which are greater than sieve size 35. Thecrush strength of Examples 1-5 is tested by compressing a proppantsample (sieved to>sieve size 35), which weighs 9.4 grams, in a testcylinder (having a diameter of 1.5 inches as specified in API RP60) for1 hour at 7500 psi and 93° C. (approximately 200° F.). Aftercompression, percent fines and agglomeration are determined.

Agglomeration is an objective observation of a proppant sample, i.e., aparticular Example, after crush strength testing as described above. Theproppant sample is assigned a numerical ranking between 1 and 10. If theproppant sample agglomerates completely, it is ranked 10. If theproppant sample does not agglomerate, i.e., it falls out of the cylinderafter crush test, it is rated 1.

TABLE 3 Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 %Fines 0.9 0.7 0.3 0.3 0.7 (<100 sieve size) % Fines 4.5 5.3 1.9 3.4 4.4(<40 sieve size) % Fines 7.1 9.7 4.1 6.1 7.7 (<35 sieve size)Agglomeration 1 1 1 1 1

Referring now to Table 3, Examples 1-5 demonstrate excellent crushstrength and agglomeration.

In addition, Example 3 is tested for conductivity and permeability inaccordance with PS-50™ fracture conductivity test, the test results areset forth in Table 4 below. International Standards Organization, ISOprocedures 13503-5 “Procedures for measuring the long term conductivityof proppants” are used to obtain baseline values. Conductivity andpermeability testing is performed at 250° F. and a stress level of6,000-psi.

TABLE 4 Stress Conductivity Permeability (psi) (mDft) (Darcy) Example 31,000 24 hrs. 4767 223 6,000 initial 2694 134 6,000 final 1488 76

Generally, the higher the conductivity and the permeability, the better.Referring now to Table 3, Example 3 demonstrates excellent conductivityand permeability.

It is to be understood that the appended claims are not limited toexpress and particular compounds, compositions, or methods described inthe detailed description, which may vary between particular embodimentswhich fall within the scope of the appended claims. With respect to anyMarkush groups relied upon herein for describing particular features oraspects of various embodiments, it is to be appreciated that different,special, and/or unexpected results may be obtained from each member ofthe respective Markush group independent from all other Markush members.Each member of a Markush group may be relied upon individually and or incombination and provides adequate support for specific embodimentswithin the scope of the appended claims.

It is also to be understood that any ranges and subranges relied upon indescribing various embodiments of the present invention independentlyand collectively fall within the scope of the appended claims, and areunderstood to describe and contemplate all ranges including whole and/orfractional values therein, even if such values are not expressly writtenherein. One of skill in the art readily recognizes that the enumeratedranges and subranges sufficiently describe and enable variousembodiments of the present invention, and such ranges and subranges maybe further delineated into relevant halves, thirds, quarters, fifths,and so on. As just one example, a range “of from 0.1 to 0.9” may befurther delineated into a lower third, i.e., from 0.1 to 0.3, a middlethird, i.e., from 0.4 to 0.6, and an upper third, i.e., from 0.7 to 0.9,which individually and collectively are within the scope of the appendedclaims, and may be relied upon individually and/or collectively andprovide adequate support for specific embodiments within the scope ofthe appended claims. In addition, with respect to the language whichdefines or modifies a range, such as “at least,” “greater than,” “lessthan,” “no more than,” and the like, it is to be understood that suchlanguage includes subranges and/or an upper or lower limit. As anotherexample, a range of “at least 10” inherently includes a subrange of fromat least 10 to 35, a subrange of from at least 10 to 25, a subrange offrom 25 to 35, and so on, and each subrange may be relied uponindividually and/or collectively and provides adequate support forspecific embodiments within the scope of the appended claims. Finally,an individual number within a disclosed range may be relied upon andprovides adequate support for specific embodiments within the scope ofthe appended claims. For example, a range “of from 1 to 9” includesvarious individual integers, such as 3, as well as individual numbersincluding a decimal point (or fraction), such as 4.1, which may berelied upon and provide adequate support for specific embodiments withinthe scope of the appended claims.

The present invention has been described in an illustrative manner, andit is to be understood that the terminology which has been used isintended to be in the nature of words of description rather than oflimitation. Obviously, many modifications and variations of the presentinvention are possible in light of the above teachings. It is,therefore, to be understood that within the scope of the appendedclaims, the present invention may be practiced otherwise than asspecifically described.

What is claimed is:
 1. A proppant for hydraulically fracturing asubterranean formation, said proppant comprising: A. a particle; and B.a polyamide imide coating disposed on said particle in an amount of fromabout 0.5 to about 7.5 percent by weight, based on 100 parts by weightof said particle.
 2. The proppant as set forth in claim 1 wherein saidparticle is selected from the group of minerals, ceramics, sands, nutshells, gravels, mine tailings, coal ashes, rocks, smelter slag,diatomaceous earth, crushed charcoals, micas, sawdust, wood chips,resinous particles, polymeric particles, and combinations thereof. 3.The proppant as set forth in claim 1 wherein said polyamide imidecoating is present in said proppant in an amount of from about 1.0 toabout 6.0 percent by weight, based on 100 parts by weight of saidparticle.
 4. The proppant as set forth in claim 1 that is thermallystable at temperatures greater than 100° C.
 5. The proppant as set forthin claim 1 having a crush strength of 15% or less maximum fines lessthan sieve size 35 as measured by compressing a 9.4 g sample of saidproppant in a test cylinder having a diameter of 1.5 inches for 1 hourat 7500 psi and 93° C.
 6. The proppant as set forth in claim 1 whereinsaid polyamide imide coating comprises the reaction product of anisocyanate and an isocyanate-reactive component.
 7. The proppant as setforth in claim 6 wherein said isocyanate comprises polymericdiphenylmethane diisocyanate and has an isocyanate (NCO) content ofabout 31.5 weight percent.
 8. The proppant as set forth in claim 6wherein said isocyanate and said isocyanate-reactive component react inthe presence of a catalyst to form said polyamide imide coating.
 9. Theproppant as set forth in claim 8 wherein said catalyst is triethylamine.10. The proppant as set forth in claim 8 wherein said isocyanate isreacted, to form said polyamide imide coating, in an amount of fromabout 0.01 to about 14 parts by weight, based on 100 parts by weight ofsaid proppant.
 11. The proppant as set forth in claim 6 wherein saidisocyanate-reactive component is further defined as the reaction productof a carboxylic acid anhydride and an amine.
 12. The proppant as setforth in claim 11 wherein said carboxylic acid anhydride comprises anaromatic carboxylic acid anhydride.
 13. The proppant as set forth inclaim 11 wherein said amine is selected from the group of polyetheramines, amino silanes, and combinations thereof.
 14. The proppant as setforth in claim 11 wherein said amine is reacted, to form said polyamideimide coating, in an amount of from about 0.001 to about 5 parts byweight, based on 100 parts by weight of said proppant.
 15. The proppantas set forth in claim 6 wherein said isocyanate-reactive componentcomprises a carboxylic acid anhydride.
 16. The proppant as set forth inclaim 15 wherein said carboxylic acid anhydride is trimelliticanhydride.
 17. The proppant as set forth in claim 15 wherein saidcarboxylic acid anhydride is reacted, to form said polyamide imidecoating, in an amount of from about 0.06 to about 14 parts by weight,based on 100 parts by weight of said proppant.
 18. A method of forming aproppant for hydraulically fracturing a subterranean formation, saidmethod comprising the steps of: A. providing a particle; B. providing apolyamide imide coating; and C. coating the particle with the polyamideimide coating particle in an amount of from about 0.5 to about 7.5percent by weight, based on 100 parts by weight of the particle.
 19. Themethod as set forth in claim 18 further comprising the step of heatingthe particle to a temperature greater than 150° C. prior to orsimultaneous with the step of coating the particle with the polyamideimide coating.
 20. The method as set forth in claim 18 furthercomprising the step of heating the proppant to further crosslink thepolyamide imide coating.
 21. The method as set forth in claim 18 furthercomprising the step of reacting an isocyanate and an isocyanate-reactivecomponent to form the polyamide imide coating.
 22. The method as setforth in claim 21 wherein the steps of reacting the isocyanate-reactivecomponent and the isocyanate to form the polyamide imide coating and ofcoating the particle with the polyamide imide coating are collectivelyconducted in 20 minutes or less.
 23. The method as set forth in claim 21wherein the step of reacting the isocyanate-reactive component and theisocyanate to form the polyamide imide coating is conducted prior to thestep of coating the particle with the polyamide imide coating.
 24. Themethod as set forth in claim 21 wherein the isocyanate-reactivecomponent comprises a carboxylic acid anhydride.
 25. The method as setforth in claim 21 further comprising the step of reacting a carboxylicacid anhydride and an amine to form the isocyanate-reactive component.26. The method as set forth in claim 21 wherein the step of reacting theisocyanate-reactive component and the isocyanate to form the polyamideimide coating is conducted simultaneous with the step of coating theparticle with the polyamide imide coating.
 27. The method as set forthin claim 26 further comprising the step of heating the proppant tofurther crosslink the polyamide imide coating.
 28. The method as setforth in claim 27 wherein the steps of reacting the isocyanate-reactivecomponent and the isocyanate to form the polyamide imide coating, ofcoating the particle with the polyamide imide coating, and of heatingthe proppant to further crosslink the polyamide imide coating arecollectively conducted in 1.5 hours or less.
 29. A method ofhydraulically fracturing a subterranean formation which defines asubsurface reservoir with a mixture comprising a carrier fluid and aproppant, the proppant comprising a particle and a polyamide imidecoating disposed on the particle in an amount of from about 0.5 to about7.5 percent by weight, based on 100 parts by weight of said particle,said method comprising the step of pumping the mixture into thesubsurface reservoir to cause the subterranean formation to fracture.30. The method as set forth in claim 29 wherein the polyamide imidecoating comprises the reaction product of an isocyanate comprisingpolymeric diphenylmethane diisocyanate and an isocyanate-reactivecomponent comprising trimellitic anhydride.